Polymethylmethacrylate sheet is widely used in thermoforming applications such as for shower/tub enclosures, outdoor signs, and the like. Such thermoforming applications require the use of large molds frequently having deep drawn areas and/or corners which bring about variations in thickness, sometimes causing a weakening of the material in areas where strength is needed. The thermoformable sheet must be easily molded and retain a good measure of strength after molding. The acrylic sheet commonly used has a high luster as well and can accommodate a wide variety of pigments, fillers, dyes, and the like. It may also contain impact modifying particles, and/or may be cross-linked for toughness. Cross-linked materials typically are made by casting rather than extrusion.
Reinforcement, in the form of unsaturated polyester resins, applied usually with glass fibers to the back of the formed acrylic sheet, is frequently necessary to guard against mechanical failures of molded articles such as shower/tub enclosures and hot tubs. The application of the fiberglass reinforced polyester resin to the individual formed product is labor-intensive and generally undesirable because of its non-uniformity and the emission of styrene during application. The art, accordingly, is in need of a strong formable acrylic sheet having a built-in reinforcement.
A review of prior patents reveals that various acrylic coatings have been applied to polystyrene substrates by the use of solvents (U.S. Pat. No. 2,801,936). In U.S. Pat. No. 2,788,051, a layer of thermosetting polymer is extruded onto a substrate. Aqueous acrylic materials are coated onto various bases including acrylic sheets in U.S. Pat. Nos. 4,066,820 and 4,202,924. Various laminated sheets and methods of making them, are disclosed in U.S. Pat. Nos. 4,717,624, 4,699,579, and 4,419,412; U.S. Pat. Nos. 4,100,325 and 4,221,836 include ABS as a component. Co-extrusion of ABS and polystyrene is shown in U.S. Pat. No. 4,100,237, and co-extrusion of two different acrylic compositions is shown in U.S. Pat. No. 3,846,368. Some of the prior art approaches to lamination involve an intermediate adhesive material, which is not necessary in applicant's process. See the complicated layering in U.S. Pat. No. 3,356,560 which involves separated layers of PMMA and a material similar to it.
J. E. Johnson, in "Co-extrusion" Plastics Technology, February, 1976, states ABS and acrylic may see wide-ranging sheet applications, largely in outdoor furniture. He states they can be readily co-extruded, and doing so combines the toughness of ABS with the weatherability of acrylic.
One of the inventors herein is aware of the commercial use of a process in about 1970 wherein a thin polyacrylate film was applied, to a substrate of a preformed ABS sheet, heated on only one side after which heat and pressure were applied in a roll stack.
The present process differs from the 1970 process in that the ABS is heated to a higher temperature and in fact is a pliant "web" emerging directly from an extruder where it is made from pellets or the like. The ABS is not only at a higher temperature, but is heated throughout rather than only on one side, thus enabling it to respond more completely to the compression environment of the roll stack, with the advantages which will be explained infra. The acrylic of the present process, being thicker than that of the 1970 process, serves as a base or substrate for the ABS. The ABS is in a more plastic state than the ABS of the prior art process, which was employed as the substrate for the acrylic film.